1. Field of the Invention
The invention relates to a method for on-vehicle functional diagnosis of a soot sensor in a motor vehicle and/or for detecting further components in the soot, and to a soot sensor operated by this method, and to an evaluation circuit permanently installed in a motor vehicle having an internal combustion engine and intended for on-vehicle functional diagnosis of a soot sensor.
2. Description of the Related Art
There is currently a lot of discussion about the accumulation of pollutants from exhaust gases in the atmosphere. Along with this, the availability of fossil fuels is limited. One response to this is to thermodynamically optimize combustion processes in internal combustion engines to improve their efficiency. In the automotive sector, this is reflected in the increasing use of diesel engines. However, the disadvantage of this combustion technology relative to optimized spark-ignition engines is a significantly increased output of soot. The absorption of polycyclic aromatic compounds makes the soot highly carcinogenic, and there have already been various regulations in response thereto. Thus exhaust emissions standards incorporating upper limits for soot emissions have been issued. There is therefore a need to specify reasonably priced sensors which reliably measure the soot content of the exhaust gas stream in motor vehicles.
Soot sensors of this kind are used to measure the soot being emitted to ensure that the engine management system in a motor vehicle receives information in a current driving situation that enable it to reduce emission levels through adaptations by control measures. Soot sensors can be used to initiate active exhaust gas cleaning by exhaust gas soot filters or to bring about recirculation of the exhaust gas to the internal combustion engine. In the case of soot filtering, filters capable of being regenerated, which filter out a significant proportion of the soot content of the exhaust gas, are used. Soot sensors are required for the purpose of detecting soot, for monitoring the operation of the soot filters and for controlling the regeneration cycles of the latter.
For this purpose, a soot sensor can be fitted upstream and/or a soot sensor can be fitted downstream of the soot filter, which is also referred to as a diesel particulate filter.
The sensor fitted upstream of the diesel particulate filter serves to increase the reliability of the system and to ensure that the diesel particulate filter operates under optimum conditions. Since this depends very largely on the mass of soot deposited in the diesel particulate filter, accurate measurement of the concentration of particulates upstream of the diesel particulate filter system, especially determination of a high particulate concentration upstream of the diesel particulate filter, is very important.
A sensor arranged downstream of the diesel particulate filter makes it possible to perform on-vehicle diagnosis and furthermore serves to ensure correct operation of the exhaust gas aftertreatment system.
There have been various approaches to the detection of soot in the prior art. One approach, which is widely followed in laboratories, is to use the scattering of light by the soot particles. This approach is suitable for complex measuring instruments. Attempts to use this as a mobile sensor system in exhaust gas as well show that approaches that involve implementing an optical sensor in a motor vehicle are associated with high costs. There are furthermore unsolved problems with regard to the soiling of the optical windows by exhaust gases from combustion.
German Laid-Open Application DE 199 59 871 A1 discloses a sensor and an operating method for the sensor, based on thermal considerations. The sensor comprises an open porous molding, e.g. a honeycomb ceramic, a heating element and a temperature detector. When the sensor is brought into contact with a volume of gas to be measured, soot is deposited thereon. For measurement, the soot deposited within a particular period of time is ignited and burnt with the aid of the heating element. The increase in temperature, which occurs as it is burnt, is measured.
Among the known particle sensors for conductive particles are those in which two or more metallic electrodes are provided, said sensors having electrodes that intermesh in the manner of a comb. Soot particles that are deposited on these sensor structures short-circuit the electrodes and thus change the impedance of the electrode structure. As the concentration of particles on the sensor surface increases, a decreasing resistance and an increasing current at a constant applied voltage can thus be measured between the electrodes. A soot sensor of this kind is disclosed in DE 10 2004 028 997 A1, for example.
In general, the comb-type electrode structure of these soot sensors is formed by thin adjacent conductor tracks. The conductor tracks are 10 μm apart, for example. In addition to the desired change in the resistance of the soot sensor caused by soot loading of the comb structure, there may also be a change in the resistance of the soot sensor due to unwanted short circuits. These unwanted short circuits can be caused by electrodes that are scratched or partially detached, for example. The measured resistance of the soot sensor would be distorted by these unwanted short circuits, and this can only be detected by regular functional diagnosis of the soot sensor.